Maybe not all of them. Ferrari and Red Bull may not have the room to manoeuvre a new diffuser in. Because they went for sidepods further back than the others in order to get their aero device-cum-mirror mount outside of the non-aero zone created to get rid of barge boards. Certainly there is some doubt that Red Bull can do it because of the suspension geometry of their rear end, Brawn alludes to this in his post-meeting interview.

I don't really have a problem with the diffusers being legal but to me it seems really weird that they ruled that way. FIA is lacking consistency. First they boast about how they're going to slow the cars down in the corners, reduce costs a lot and make overtaking easier. Now they rule the new diffusers legal, a decision which increases cornering speeds, increases costs for every team that has to redesign their diffuser and rear of the car, and makes overtaking more difficult because the diffusers will give out more turbulent air behind the car. It's just stupid.

They haven't only just ruled them legal - they've always been legal, so it's no change at all (which is arguably more consistent than ruling them illegal). There should be no increased costs, no increased cornering speeds or more difficult overtaking, because this has always been the rule.

What the teams should have done is listen to Ross Brawn when he suggested tightening up the regulations in that area so that they would fit more closely with the intentions of slowing cars down (but not reduce really costs- running without split level diffusers is just as expensive as running with them, it's only if you have to change what you've already developed that it costs more).